The availability of several techniques and devices for identifying the geographical location of mobile device users has enabled a large variety of commercial and non-commercial services, such as navigation assistance, enhanced social networking, location-aware advertising, and location-aware emergency calls. However, different services may have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, such as the FCC's E-911-related requirements in the United States.
In many environments, the position of a mobile device user can be accurately estimated by using positioning methods based on GPS (Global Positioning System) or other satellite-based system. Nowadays, wireless networks are often able to provide positioning-related assistance to mobile terminals (often referred to as user equipment, or UEs, or wireless terminals, mobile stations, or simply “mobiles”) to improve the terminal's receiver sensitivity and GPS start-up performance. Several of these techniques are known as Assisted-GPS positioning, or A-GPS.
GPS or A-GPS receivers may not be available in all UE, however. Furthermore, GPS is known to fail in certain indoor environments and in urban “canyons” in the radio shadows caused by tall buildings. Complementary terrestrial positioning methods, such as one approach called Observed Time-Difference-of-Arrival (OTDOA), have therefore been standardized by the 3rd-Generation Partnership Project (3GPP) and are deployed in various wireless networks. In addition to OTDOA, the 3GPP standards for the so-called Long-Term Evolution (LTE) wireless system also specify methods, procedures and signalling support for techniques called Enhanced Cell ID (E-CID) and Assisted Global Navigation Satellite System (A-GNSS). Later, a network-based technique called Uplink Time-Difference-of-Arrival (UTDOA) may also be standardized for LTE.
Three key network elements for providing location services (LCS) in an LTE positioning architecture include the LCS Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e. the entities being positioned. LCS Clients may reside in the LCS targets themselves. An LCS Client sends a request to LCS Server to obtain location information, and LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client.
Position calculation can be conducted, for example, by a UE or by a positioning server, such as an Enhanced Serving Mobile Location Center, E-SMLC, or Secure User Plan Location (SUPL) Location Platform (SLP) in LTE. The former approach corresponds to the UE-based positioning mode, whilst the latter corresponds to the UE-assisted positioning mode.
Two positioning protocols operating via the radio network exist in LTE, LTE Positioning Protocol (LPP) and LPP Annex (LPPa). The LPP is a point-to-point protocol between a LCS Server and a LCS target device, used in order to position the target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel thereby reducing latency. LPPa is a protocol between evolved Node B (eNodeB) and LCS Server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. SUPL protocol is used as a transport for LPP in the user plane. LPP has also a possibility to convey LPP extension messages inside LPP messages, e.g. currently Open Mobile Alliance (OMA) LPP extensions are being specified (LPPe) to allow e.g. for operator-specific assistance data or assistance data that cannot be provided with LPP or to support other position reporting formats or new positioning methods.
A high-level architecture of such an LTE system 10 is illustrated in FIG. 1. In FIG. 1, the system 10 includes a UE 12, a radio access network (RAN) 14, and a core network 16. The UE 12 comprises the LCS target. The core network 16 includes an E-SMLC 18 and/or an SLP 20, either of which may comprise the LCS Server. The control plane positioning protocols with the E-SMLC 14 as the terminating point include LPP, LPPa, and LCS-AP. The user plane positioning protocols with the SLP 16 as the terminating point include SUPL/LPP and SUPL. Although note shown, the SLP 20 may comprise two components, a SUPL Positioning Center (SPC) and a SUPL Location Center (SLC), which may also reside in different nodes. In an example implementation, the SPC has a proprietary interface with E-SMLC, and an Llp interface with the SLC. The SLC part of the SLP communicates with a P-GW (PDN-Gateway) 22 and an External LCS Client 24.
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons 26 is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.
To meet varying demands for different Location-Based Services (LBS), an LTE network will deploy a range of complementing methods characterized by different performance in different environments. Depending on where the measurements are conducted and where the final position is calculated, the methods can be UE-based, UE-assisted, or network-based, each with own advantages. The following methods are available in the LTE standard for both the control plane and the user plane: (1) Cell ID (CID), (2) UE-assisted and network-based E-CID, including network-based angle of arrival (AoA), (3) UE-based and UE-assisted A-GNSS (including A-GPS), and (4) UE-assisted OTDOA.
Several other techniques such as hybrid positioning, fingerprinting positioning and adaptive E-CID (AECID) do not require additional standardization and are therefore also possible with LTE. Furthermore, there may also be UE-based versions of the methods above, e.g. UE-based GNSS (e.g. GPS) or UE-based OTDOA, etc. There may also be some alternative positioning methods such as proximity based location. UTDOA may also be standardized in a later LTE release, since it is currently under discussion in 3GPP. Similar methods, which may have different names, also exist for radio-access technologies (RATs) other than LTE, such as CDMA, WCDMA or GSM.
With particular regard to the OTDOA positioning method, this method makes use of the measured timing of downlink signals received from multiple base stations (evolved NodeBs, or eNodeBs, in LTE) at the UE. The UE measures the timing of the received signals using assistance data received from the LCS server, and the resulting measurements are used to locate the UE in relation to the neighbouring eNodeBs.
More specifically, the UE measures the timing differences for downlink reference signals received from multiple distinct locations or neighboring cells. For each (measured) neighbor cell, the UE measures Reference Signal Time Difference (RSTD), which is a relative timing difference between the neighbor cell and a defined reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the UE and the receiver clock bias. In order to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning (positioning reference signals, or PRS) have been introduced and low-interference positioning subframes have been specified in 3GPP. Details are specified in 3GPP TS 36.211; as of February 2011, version 10.0.0 of this specification is available from http://www.3gpp.org.
PRS are transmitted from one antenna port of a base station according to a pre-defined pattern. A frequency shift, which is a function of Physical Cell Identity (PCI), can be applied to the specified PRS patterns to generate orthogonal patterns. The mapping of frequency shifts to PCT models an effective frequency reuse of six, which makes it possible to significantly reduce neighbor cell interference on the measured PRS and thus improve positioning measurements. Even though PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g. cell-specific reference signals (CRS) could be used for positioning measurements, in principle.
PRS are transmitted in pre-defined positioning sub-frames grouped by several consecutive sub-frames (NPRS), i.e., one positioning occasion. Positioning occasions occur periodically with a certain periodicity of N subframes, i.e. the time interval between two positioning occasions. The standardized periods N are 160, 320, 640, and 1280 ms, and the number of consecutive subframes may be 1, 2, 4, or 6 [3GPP TS 36.211]. PRS configuration and PRS offset from System Frame Number 0 (SFN0) are determined by a PRS configuration index defined in [3GPP 36.211] and signalled in the OTDOA assistance data. The number of consecutive DL subframes and the PRS bandwidth (which may be smaller than the system bandwidth) may also be signalled in the OTDOA assistance data. Of course, signaling the PRS bandwidth in the assistance data is only useful if RSTD measurements are performed on PRS (as opposed to other reference signals).
PRS may also be muted, e.g., not transmitted. The positioning node informs the UE about whether PRS is muted or not, e.g., by signalling a cell-specific muting pattern which indicates PRS positioning occasions in which the UE is expected to perform measurements for the corresponding cell.
Information about such PRS and other information that will assist with positioning measurements is included in so-called assistance data. Different sets of assistance data are typically used for different methods. Regardless, the positioning assistance data is sent by the positioning server, or via some other node, to UEs or other radio nodes in order to assist with positioning measurements. For example, assistance data may be sent via LPP to an eNodeB for transmission to the UE. In this case, the transmission of assistance data may be transparent to the eNodeB and the Mobility Management Entity (MME). The assistance data may also be sent by the eNodeB via LPPa to a positioning server for further transfer to the UE. In some cases, the assistance data may be sent on request from a wireless device that needs to perform measurements. In other cases, the assistance data is sent in an unsolicited way.
Since for OTDOA positioning PRS signals from multiple distinct locations need to be measured, the UE receiver may have to deal with PRS that are much weaker than those received from the serving cell. Furthermore, without an approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE must perform signal search within a large window. This can impact the time and accuracy of the measurements as well as the UE complexity. To facilitate UE measurements, the network transmits assistance data to the UE, which includes, among other things, reference cell information, a neighbour cell list containing Physical Cell Identifiers (PCIs) of neighbour cells, the number of consecutive downlink subframes within a positioning occasion, PRS transmission bandwidth, frequency, etc.
In LPP, the OTDOA assistance data is provided within the Information Element (IE) OTDOA-ProvideAssistanceData, as shown in FIG. 2. Similar structures for OTDOA exist in LPPe.
The OTDOA assistance data includes information about the reference cell and neighbour cells for which OTDOA is to be determined. The neighbour cells may or may not be on the same frequency as the reference cell, and the reference cell may or may not be on the same frequency as the serving cell, and may or may not be the serving cell. Measurements that involve cells on a frequency different than the serving cell are inter-frequency measurements. Measurements on the same frequency as the serving cell are intra-frequency measurements. Different requirements apply for intra- and inter-frequency measurements.
Note that assistance data delivery is not required for UE- or eNodeB-assisted forms of E-CID positioning and this is not currently supported without EPDU elements. UE-based E-CID location is not currently supported, and the assistance data delivery procedure is not applicable to uplink E-CID positioning. No assistance data is currently specified for E-CID for LPP. Some assistance data, however, may be provided for E-CID e.g. via LPPe.
In this regard, with Open Mobile Alliance (OMA) LPP extension (LPPe), assistance data is enhanced with the possibility to assist a larger range of positioning methods (e.g. assistance data may also be provided for E-CID or other methods of other RATs, e.g. OTDOA UTRA or E-OTD GSM, or other PLMN networks). Furthermore, there is also a possibility of carrying over a black-box data container meant for carrying vendor-/operator-specific assistance data.
Also note that LTE specifications enable Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation modes. Additionally, half duplex operation is also specified, which is essentially FDD operation mode but with transmission and receptions not occurring simultaneously as in TDD. Half duplex mode has advantages with some frequency arrangements where the duplex filter may be unreasonable, resulting in high cost and high power consumption. Since carrier frequency number (EARFCN) is unique, by knowing it, it is possible to determine the frequency band, which is either FDD or TDD. However, it may be more difficult to find the difference between full duplex FDD and half-duplex FDD (HD-FDD) without explicit information since the same FDD band can be used as full FDD or HD-FDD.
Further, inter-frequency measurements may in principle be considered for any positioning method, even though currently not all measurements are specified by the standard as intra- and inter-frequency measurements. When performing inter-frequency measurement, the serving and target carrier frequencies may belong to the same duplex mode or to different duplex modes e.g. LTE FDD-FDD inter-frequency, LTE TDD-TDD inter-frequency, LTE FDD-TDD inter-frequency or LTE TDD-FDD inter-frequency scenario. The FDD carrier may operate in full duplex or even in half duplex mode. Examples of inter-frequency measurements currently specified by the standard are Reference Signal Time Difference (RSTD) used for OTDOA, RSRP and RSRQ which may be used e.g. for fingerprinting or E-CID.
In LTE, measurement gaps are configured by the network to enable inter-frequency measurements on the other LTE frequencies. The measurements may be done for various purposes: mobility, positioning, self organizing network (SON), minimization of drive tests, etc. Regardless, the gap configuration is signaled to the UE over the Radio Resource Control (RRC) protocol as part of the measurement configuration. A UE that requires measurement gaps for positioning measurements, e.g., OTDOA, may send an indication to the network, e.g. eNodeB, upon which the network may configure the measurement gaps. Furthermore, the measurement gaps may need to be configured according to a certain rule, e.g. inter-frequency RSTD measurements for OTDOA require that the measurement gaps are configured according to the inter-frequency requirements in 36.133, Section 8.1.2.6, e.g. not overlapping with PRS occasions of the serving carrier and using gap pattern #0.
In LTE, inter-RAT measurements (e.g., measurements on other RATs like UTRA, GSM, CDMA2000, etc) are typically defined similar to inter-frequency measurements. Indeed, they may also require configuring measurement gaps like for inter-frequency measurements. Although inter-RAT measurements often have more relaxed requirements and have more measurements restrictions, the same gap pattern is used for all types of inter-frequency and inter-RAT measurements. Therefore E-UTRAN must provide a single measurement gap pattern with constant gap duration for concurrent monitoring (i.e. cell detection and measurements) of all frequency layers and RATs.
As a special example of inter-RAT measurements there may also be multiple networks, which use the overlapping sets of RATs. The examples of inter-RAT measurements specified currently for LTE are UTRA FDD CPICH RSCP, UTRA FDD carrier RSSI, UTRA FDD CPICH Ec/No, GSM carrier RSSI, and CDMA2000 1x RTT Pilot Strength.
For positioning, assuming that LTE FDD and LTE TDD are treated as different RATs, the current standard defines inter-RAT requirements only for FDD-TDD and TDD-FDD measurements, and the requirements are different in the two cases. There are no other inter-RAT measurements specified within any separate RAT for the purpose of positioning and which are possible to report to the positioning node (e.g. E-SMLC in LTE).
It is mandatory for all UEs to support all intra-RAT measurements (including both inter-frequency and intra-band measurements) and meet the associated requirements. However the inter-band and inter-RAT measurements are UE capabilities, which are reported to the network during the call setup. The UE supporting certain inter-RAT measurements should meet the corresponding requirements. For example a UE supporting LTE and WCDMA should support intra-LTE measurements, intra-WCDMA measurements and inter-RAT measurements (i.e. measuring WCDMA when serving cell is LTE and measuring LTE when serving cell is WCDMA). Hence network can use these capabilities according to its strategy. These capabilities are highly driven by factors such as market demand, cost, typical network deployment scenarios, frequency allocation, etc.
Notably, in single carrier LTE, a cell may operate at channel bandwidths ranging from 1.4 MHz to 20 MHz. However, a single-carrier legacy UE shall be able to receive and transmit over 20 MHz, i.e., the maximum single-carrier LTE bandwidth. If the serving-cell bandwidth is smaller than 20 MHz, then the UE shortens the bandwidth of its radio frequency (RF) front end. For example, if the serving-cell bandwidth (BW) is 5 MHz, then the UE will likewise configure its RF BW to 5 MHz. This approach has several advantages. For example, it enables the UE to avoid noise outside the current reception bandwidth, and it saves UE battery life by lowering power consumption.
However, such reconfiguration of the UE reception and/or transmission bandwidth involves some delay, e.g., 0.5-2 ms, depending on UE implementation and also on whether both UL BW and DL BW are reconfigured at the same time or not. This small delay is often referred to as ‘glitch’. During the glitch the UE cannot receive from the serving cell or transmit to the serving cell. Hence this may lead to interruption in data reception or transmission from or to the serving cell. The UE is also unable to perform any type of measurements during the glitch. The glitch occurs either when the UE extends its bandwidth (e.g. from 5 MHz to 10 MHz) or when it shortens its bandwidth (e.g. from 10 MHz to 5 MHz).
Furthermore, when the UE operates at a bandwidth lower than its maximum reception capability and the UE then wants to measure over a larger bandwidth, it has to open its receiver for performing the measurement. Thus, in this case (i.e. when current BW<max BW) the glitch occurs before and after the UE obtains each measurement sample, if the UE reconfigures back to its current operation after each measurement sample over the larger bandwidth.
The glitch also occurs when a UE capable of carrier aggregation (CA) reconfigures its bandwidth from single carrier to multiple carrier mode or vice versa. For example consider a UE that is capable of CA and that supports 2 downlink (DL) component carriers (CCs), each of 20 MHz, including a primary CC (PCC) and a secondary CC (SCC). If the secondary component carrier is deactivated by the serving/primary cell then the UE will shorten its BW e.g. from 40 MHz to 20 MHz. This may cause 1-2 ms interruption on the PCC.
According to current standards, the maximum allowed measurement bandwidth on a carrier frequency is defined by the parameter Transmission Bandwidth Configuration “NRB” in 3GPP TS 36.104, which may take values of 6, 15, 25, 50, 75 and 100 resource blocks. The DL bandwidth information of a cell is signaled in the Mater Information Block (MIB) which the UE reads before it can camp on the cell; the UL bandwidth information, if different from the DL bandwidth information, may further be signaled in SystemInformationBlockType2 (SIB2) [3GPP TS 36.331].
For cell reselection, i.e., when the UE has to measure on neighbor cells, the cell re-selection parameters that are common for intra-frequency, inter-frequency and/or inter-RAT cell re-selection are signaled in SystemInformationBlockType3 (SIB3). The element intraFreqCellReselectionInfo of SIB3 contains the allowedMeasBandwidth element, which corresponds to the DL bandwidth for measurements on intra-frequency cells. If that element is absent, the DL measurement bandwidth for intra-frequency cells is assumed to be the same as that indicated by the dl-Bandwidth included in MIB. The allowed measurement bandwidth is not signaled per cell, since it is assumed to be the same as for the serving cell, which is signaled in MIB and SIB2.
The information relevant for inter-frequency cell reselection only may be signaled via SIB5, which includes cell re-selection parameters common for a frequency as well as cell specific re-selection parameters. The allowed measurement bandwidth information is signaled per frequency in the InterFreqCarrierFreqInfo element.
Thus, cell-specific bandwidth information currently is not provided for cell re-selection. Rather, bandwidth information for cell re-selection is only provided per carrier.
Other cell-specific information for cell re-selection is currently provided for intra-frequency cells or inter-frequency cells. For intra-frequency cells, the information is provided in the IntraFreqNeighCellInfo element, when a list of cells is signaled in SIB4. For inter-frequency cells, the information is provided in the InterFreqNeighCellInfo element, when a list of cells is included in InterFreqCarrierFreqInfo signaled in SIB5.
Further, a neighCellConfig element is used to indicate whether or not some configurations for a neighbor cell are the same as for the serving cell. This element with the current standard can be signaled as either a part of intraFreqCellReselectionInfo (in SIB3) or a part of InterFreqCarrierFreqInfo (in SIB5).
Note that the neighCellConfig element is used to indicate potential configuration differences among cells of a particular frequency, without cell details. Currently, the neighCellConfig element is used to provide only the information related to MBSFN and TDD UL/DL configuration of neighbour cells of such frequency. In particular, values for the neighCellConfig element include 00, 10, 01, and 11. A value of ‘00’ indicates that not all neighbour cells have the same MBSFN subframe allocation as the serving cell on the frequency, if configured, and as the PCell otherwise. A value of ‘10’ indicates that the MBSFN subframe allocations of all neighbour cells are identical to or subsets of that in the serving cell on this frequency, if configured, and of that in the PCell otherwise. A value of ‘01’ indicates that no MBSFN subframes are present in all neighbour cells. Finally, a value of ‘11’ indicates that there is a different UL/DL allocation in neighbouring cells for TDD compared to the serving cell on this frequency, if configured, and compared to the PCell otherwise. Note that, for TDD, 00, 10 and 01 are only used for the same UL/DL allocation in neighbouring cells compared to the serving cell on this frequency, if configured, and compared to the PCell otherwise.
In view of the above described details, a UE may need to measure reference signals transmitted by multiple cells, e.g., for performing positioning measurements. This proves problematic in certain circumstances. One problematic circumstance occurs when the multiple cells have different cell bandwidths. Another problematic circumstance occurs when one or more of the cells do not use the full cell bandwidth, such as when those cells are provided by beacon devices. Still another problematic circumstance occurs when the reference signals to be measured are transmitted in the multiple cells with different bandwidths (irrespective of the cell bandwidths of those cells). And yet another problematic circumstance occurs when the UE obtains different measurement bandwidth information for cells to be measured and thereby measures those cells over different bandwidths.
In all of these circumstances, the UE has to reconfigure the receiver to enable measurements of cells with a larger bandwidth, which may be necessary to meet e.g. measurement accuracy requirements with respect to those cells. This proves problematic because configuring a receiver to a larger bandwidth, to meet measurement accuracy requirements for cells with that larger bandwidth, may degrade measurement quality in other cells with either a smaller associated measurement bandwidth or with a smaller cell bandwidth. Configuring the receiver to a larger bandwidth may also prove problematic if that bandwidth is larger than the serving-cell bandwidth. Indeed, particularly where the measurements being performed are intra-frequency measurements, measuring cells over such large bandwidth degrades the quality with which the UE receives data from the serving cell over a smaller bandwidth.
Still further, positioning measurements may be performed periodically. For instance, OTDOA positioning measurements are performed in positioning subframes that occur in blocks of consecutive DL subframes and with periodicity of 160 ms, 320, 640 ms, or 1280 ms. Receiver reconfiguration to a new measurement bandwidth in certain subframes takes time, and reconfiguring it back to the normal-operation measurement bandwidth in normal subframes also takes time. This reconfiguration time reduces the total effective measurement time, which typically results in degraded measurement accuracy and/or data reception quality.
Moreover, when the network (eNodeB in LTE) configures measurement gaps for the UE to enable positioning measurements, there may also be some cells on inter-frequency(ies) or another RAT with a different transmission or measurement bandwidth of signals used for positioning. In some cases, e.g., when there are multiple frequencies and the signals for positioning occur at different time instances, the network (or eNodeB, in particular) may need to choose for which frequency the measurement gaps are to be configured. Known approaches fail to make a selection in this regard that would improve measurement accuracy and/or data reception quality.